The present invention relates to a linear-circular polarizer for receiving electro-magnetic waves in the microwave band used in satellite broadcasting.
In FIG. 13 and FIG. 14, a prior art linear-circular polarizer 70 is shown. FIG. 13 is a front view of the linear-circular polarizer viewed from the opening of the linear-circular polarizer. FIG. 14 is a cross-sectional view of the linear-circular polarizer of FIG. 13 along the line 14--14. The linear-circular polarizer includes a waveguide having a circular hollow shape with a circular wall surface and a 1/4 wavelength phase plate 1.
The 1/4 wavelength phase plate 1 is made of a metallic material and has a flat trapezoidal shape with a specified sloping surface 1A (shown in FIG. 14) provided at each end. This provides excellent values for both the impedance from a primary radiator 11 towards the phase plate 1 (input impedance) and the impedance from an excitation slot 12 (see FIG. 13) towards the phase plate 1 (output impedance). The phase plate 1 has a specified width (plate thickness) and a flat mounting surface (joining surface shape). This phase plate 1 is attached to the inner surface of circular waveguide 6 (as shown in FIG. 14) at a position which forms an opening angle of 45 degrees from the horizontal axis. Plate 1 is positioned along the axis of the circular waveguide 6 by means of screws 5. Where the phase plate 1 joins the circular waveguide 6, a space exists between the inner surface of the circular waveguide 6 and the surface of phase plate 1. Only the outside edges of the phase plate 1 are in contact with the circular waveguide 6 as shown in a magnified view of the j area where phase plate 1 contacts circular waveguide 6. (See the right section of FIG. 13.) FIG. 10 and FIG. 11 show cross-polarization discrimination characteristics and input impedance characteristics including the characteristics of the linear-circular polarizer shown in FIG. 13 and FIG. 14.
Another prior art example appears in the Utility Model Gazette "Sho 59-108032" of Japan. A linear-circular polarizer comprises four ridges (referred to herein as phase plates) of the same width and height that are disposed on the inner electro-conductive walls of a circular waveguide and arranged 90 degrees apart from one another around the waveguides axis. The flat phase plates are formed of a dielectric material and inserted so as to overlay a pair of the ridges which are symmetric with each other about the waveguides axis. According to this prior art, a circular polarized wave is converted to a linear polarized wave by means of a phase plate formed of a dielectric material. The four ridges are intended for widening the bandwidth characteristics of the waveguide but do not correspond to a linear-circular polarizer.
According to the foregoing prior art structures, the minimum thickness of the phase plate 1 is restricted by the diameter of the mounting screw 5 which makes achieving optimum performance difficult. In addition, the phase plate 1 has sloping sections 1A, thereby making it impossible to remove a male die from the primary radiator side in the course of fabrication and to employ injection molding (aluminum die-cast, for example) as the production method. Therefore, the phase plate 1 has to be attached to the inside of the waveguide 6 as a separate piece.
Further, according to the prior art method, the junction surface of the circular waveguide is concave while the junction surface of the phase plate 1 is flat. This results in an imperfect ground connection due to extremely small contacting areas between the phase plate 1 and the waveguide 6 and a large variation in the mounting position of the phase plate 1.
Consequently, it has been difficult for the prior art structures to achieve excellent impedance characteristics and cross polarization characteristics.
Small errors in the mounting position of the phase plate 1 cause great deterioration in the cross polarization characteristics, thereby making it difficult to achieve stabilized performance. Because of this, frequent correction of the joining position of the phase plate 1 was necessary during mass-production.